Inkjet recording method has been rapidly spread and further developed because it requires a reduced material cost, allows a high speed recording, causes little noise during recording and allows easy color recording.
Examples of inkjet recording method include a continuous method involving continuous ejection of droplets and an on-demand method involving ejection of droplets according to an image data signal. Examples of ejection method include a method which allows a piezoelectric element to give pressure that causes a droplet to be ejected, a method which comprises heating the ink to generate bubbles, causing a droplet to be ejected, a method involving the use of ultrasonic wave, and a method which uses electrostatic force to suck and discharge a droplet. As inks for these inkjet recording methods there are used aqueous inks, oil-based inks and solid (melt type) inks.
The colorants to be incorporated in these inks for inkjet recording are required to exhibit a high solubility in solvents, allow a high density recording and have a good hue and an excellent fastness to light, heat, active gas in the atmosphere (e.g., oxidizing gas such as NOx and ozone, SOx), water and chemical, a good fixability to image-receiving materials, difficulty in bleeding, an excellent preservability, no toxicity and a high purity and be available at a low cost.
However, it is extremely difficult to find dyes meeting all these requirements to a high extent. In particular, there have been keenly desired colorants having a good hue, a high fixability to the image recording material even under high humidity conditions a high fastness to light and active gases in the atmosphere, particularly oxidizing gas such as ozone.
In recent years, it has been practiced to form a color image and obtain a recorded matter by an inkjet recording method using a plurality of color ink compositions. In general, a color image is formed by four color ink compositions, i.e., yellow ink composition, magenta ink composition and cyan ink composition, and optionally a black ink composition. Further, in some cases, a color image is formed by six color ink compositions, i.e., light cyan ink composition and light magenta ink composition in addition to the four color ink compositions. In further cases, a color image is formed by seven color ink compositions, i.e., dark yellow ink composition in addition to the six color ink compositions. A combination of two or more of these ink compositions is called an ink set.
The aforementioned ink compositions to be used in the formation of a color image need to meet requirements that they each have a good color developability, any combination of a plurality of ink compositions can develop a good intermediate color and the recorded matter obtained therefrom undergoes no fading during storage.
When the light-resistance or ozone resistance of a specific ink composition in the ink set is remarkably lower than that of the other ink compositions, the color developed from the specific ink composition fades faster than the other colors, causing the deterioration of tone balance of the entire image. In addition to the requirement that the light-resistance or ozone resistance of the various ink compositions constituting the ink set be enhanced, a requirement is preferably met that the rate of deterioration such as fading of the images formed by the various ink compositions be as the same as possible. In other words, it is desirable that the various ink compositions constituting the ink set be excellent in light-resistance or ozone resistance and have a small difference in light-resistance or ozone resistance among themselves.
A representative example of the skeleton of cyan dye used in ink for inkjet recording is a phthalocyanine dye. In recent years, phthalocyanine dyes having improved light-resistance and ozone resistance have been proposed in JP-A-2002-285050, JP-A-2002-309118 and JP-A-2004-002670.
On the other hand, representative examples of the skeleton of magenta dye used in ink for inkjet recording include xanthene dyes disclosed in JP-A-8-60053, H-acid azo dyes disclosed in JP-A-7-157698, and anthrapyridone dyes and heterocyclic-heterocyclic azo dyes disclosed in JP-A-57-195775, JP-A-59-074173, JP-A-10-306221 and JP-A-2000-191660.
Among these dyes, the xanthene dyes are excellent in hue and sharpness but leave something to be desired in light-resistance and ozone resistance. The H-acid azo dyes are excellent in hue or water resistance but leave something to be desired in light-resistance, ozone resistance and sharpness. The anthrapyridone dyes are excellent in sharpness and light-resistance but leave something to be desired in ozone resistance. In recent years, however, anthrapyridone dyes having improved ozone resistance have been disclosed in JP-A-2003-192930, International Publication No. 04/104108, etc. but still leave something to be desired.
Further, heterocyclic-heterocyclic azo dyes excellent in light-resistance and ozone resistance have been proposed in JP-A-2002-371079, JP-A-2002-371214, etc. but leave something to be desired in fixability to the image-receiving material under high humidity conditions.